Summary. Laboratory bred bank voles of different age and reproductive status were bilaterally ovariectomized, treated with exogenous oestrogen or progesterone or both, and given an artificial decidual stimulus (crushing with artery forceps). An hormonal regimen of oestrogen (100 ng) followed by progesterone (1 mg) and small amounts of oestrogen (10 ng) was necessary for maximal development of the decidual cell reaction (DCR). A DCR did not develop if the animals were treated with only oestrogen or progesterone. Middle-aged virgin females 5–7 months old had larger responses than did young virgin females 2–3 months old, and parous females, 15–19 months old and having had at least 10 litters, had a response similar to that of virgin females of about the same age. The size of the DCR in old parous females was inversely related to the time elapsing since the last litter. It is concluded that in its hormonal requirements for a successful DCR the bank vole is similar to the mouse. The results also suggest that the increase in fertility of bank voles up to the age of about 100 days may in part be explained by increased response of the endometrium to blastocysts. The reduction in size of the DCR in old animals is a consequence of ageing rather than of repeated pregnancies.